1. Field of the Invention
The present invention relates to a multi-band inductor-capacitor (LC) resonance voltage-controlled oscillator (VCO) and, more specifically, to a multi-band LC resonance voltage-controlled oscillator that operates in a multi-band multi-mode and compensates for fluctuation in the amplitude of an oscillation wave.
2. Discussion of Related Art
It is difficult to embody a wide-band LC resonance voltage-controlled oscillator in a multi-band multi-mode wireless transceiver. Nowadays, capacitor banks 122-1 and 122-2 and varactors 121 are mounted in an LC resonance portion with a fixed inductor 130 as shown in FIG. 1, in order to extend the frequency adjustment range of the wide-band LC resonance voltage-controlled oscillator.
Although FIG. 1 shows capacitors of the capacitor banks 122-1 and 122-2 arranged to increase as powers of two in the LC resonance voltage-controlled oscillator, they may be arranged as powers of any positive integer or in various other manners. Thus, the capacitance of the capacitor banks 122-1 and 122-2 is determined by control bit signals V0, V1, V2, and V3 of FIG. 1 so that a signal with a desired frequency can be generated by the LC resonance voltage-controlled oscillator of FIG. 1.
However, since the capacitance of the capacitor banks 122-1 and 122-2 is dependent on the control bit signals V0, V1, V2, and V3, when the oscillation signal frequency and amplitude of the LC resonance oscillator are different or, in the worst case, when the capacitance of the selected capacitor banks 122-1 and 122-2 is too high, oscillation may cease to occur. This phenomenon results from non-uniform oscillation amplitude and oscillation stop, which arise due to a certain load driving current of the oscillator as well as variation in the capacitance of the capacitor banks 122-1 and 122-2 in response to the control bit signals V0, V1, V2, and V3. In order to prevent non-uniform oscillation amplitude and oscillation stop, a plurality of tail current sources 190 for LC resonance are connected in parallel as shown in FIG. 1. The tail current sources 190 are turned on/off in response to the control bit signals V0, V1, V2, and V3 that determine the capacitance of the capacitor banks 122-1 and 122-2. The following Equation 1 expresses a noise element of the oscillator:
                              F          =                      1            +                                          4                ⁢                                                                  ⁢                γ                ⁢                                                                  ⁢                IR                                            π                ⁢                                                                  ⁢                                  V                  s                                                      +                                          4                ⁢                                                                  ⁢                γ                ⁢                                                                  ⁢                                  g                  m                                ⁢                                                                  ⁢                R                            9                                      ,                            (        1        )            wherein “g” denotes a channel noise coefficient, “I” denotes a bias current, “Vs” denotes an oscillation signal amplitude, “R” denotes an LC resonance load resistance, and “gm” denotes the transconductance of a transistor of the tail current source 190. From Equation 1, it can be seen that the noise element “F” of the oscillator is proportional to the bias current “I” and the transconductance “gm” and inversely proportional to the oscillation signal amplitude “Vs.”
As described above, a band switching technique using capacitors or inductors has been used in order to extend the frequency adjustment range of the LC resonance voltage-controlled oscillator. In this case, fluctuation in amplitude of the oscillation frequency signal occurs relative to the capacitance of a switching capacitor. In order to compensate for this fluctuation in the oscillation amplitude, the current of the tail current source 190 is increased. However, the tail current source 190 brings about low- and high-frequency thermal noise, thus increasing the phase noise of the LC resonance voltage-controlled oscillator.